Equilibria and dynamics of some aqueous peroxomolybdophosphate catalysts: a potentiometric and31P NMR spectroscopic study

Abstract

The equilibrium speciation in the pH+ + qMoO42− + rH2O2 + sH2PO4− + tSO42− ⇌ (H+)p(MoO42−)q(H2O2)r(H2PO4−)s(SO42−)tp−2q−s−2t system in 0.300 M Na2(SO4) medium at 25 °C and [H2O2]tot/[Mo]tot > 2 has been determined from potentiometric data and 31P NMR integral and chemical shift data in the range 1.0 ≤ pH ≤ 9.0, 2.0 ≤ [MoO42−]tot/mM ≤ 293, 32 ≤ [H2O2]tot/mM ≤ 640, 2 ≤ [H2PO4−]tot/mM ≤ 60 and 200 ≤ [SO42−]tot/mM ≤ 380. Species with the following compositions were obtained: MoX2P3− (0,1,2,1,0), MoX2P2− (1,1,2,1,0), MoX2P− (2,1,2,1,0), Mo2X4P3− (2,2,4,1,0), Mo2X4P2− (3,2,4,1,0), Mo3X6P3− (4,3,6,1,0), Mo3X6P2− (5,3,6,1,0) and Mo4X8P3− (6,4,8,1,0). The numbers and charges of molybdenum (Mo), peroxide (X), phosphate (P) and sulfate (S) in each species are given in the abbreviated formula MoqXrPsStp−2q−s−2t. The numbers in parentheses refer to the values of p, q, r, s and t in the formula above. The following formation constants with 3σ were obtained; log β0,1,2,1,0 = 5.16 ± 0.09, log β1,1,2,1,0 = 12.73 ± 0.02 (pKa = 7.63), log β2,1,2,1,0 = 16.14 ± 0.03 (pKa = 3.42), log β2,2,4,1,0 = 25.03 (± 0.04), log β3,2,4,1,0 = 29.54 ± 0.02 (pKa = 4.51), log β4,3,6,1,0 = 42.30 ± 0.03, log β5,3,6,1,0 = 44.06 ± 0.08 (pKa = 1.76), log β6,4,8,1,0 = 57.30 ± 0.07. pKa values for H3PO4 and H2PO4− were determined to 2.00 and 6.48, respectively. Chemical exchange processes were detected by 31P NMR 2D EXSY and selective magnetisation transfer experiments between the complexes. An averaged lifetime, τ(X2Mo–OP) ∼ 30 ms, can be estimated for the MoX2P and Mo2X4P species at pH = 5.5 and 5 °C.